This invention relates to a tachometer generator and particularly, to such a device of the inductive pickup variety adapted to provide an electrical output signal responsive to a changing magnetic field.
Tachometer generators are employed in widespread applications with mechanical devices. Tachometer generators are used, for example, to measure the speed of rotation of a shaft, to sense shaft movement, or to provide an indication of the direction of rotation of a shaft. One type of tachometer generator is the so-called inductive pickup variety. These devices are used in connection with a rotating magnet, usually of the permanently magnetized variety. As the magnet rotates in proximity with a magnetic core piece of the tachometer generator, a closed magnetic circuit is created. Positioned around the core member of the generator is a coil having multiple turns of conductive wire. In accordance with Faraday's Law, an induced electromotive force (voltage or e.m.f.) is produced when the coil of wire is exposed to a magnetic flux which changes with respect to time. The magnitude of the induced voltage is proportional to the number of turns of wire used, the magnetic flux density, and the rate of change of that magnetic flux. The intensity of the flux is a function of the magnetic circuit path and the magnetic characteristics (reluctance) of the materials within the magnetic circuit path. By employing a rotating multipole permanent magnet positioned in close proximity with the stationary tachometer generator device, an alternating current output is produced as the core member is exposed to alternating poles of the magnet as it is rotated. This alternating current output can be processed according to well-known principles to generate a signal which is a function of the angular speed of the rotating magnet.
Tachometer generators of the inductive variety are well-known according to the art. This invention, however, improves over prior art designs by providing a structure which is simple in construction, reliable and inexpensive to manufacture. Such advantages are particularly desirable when using such devices with portable electric tools such as drill motors, with which this invention is particularly advantageously employed. These advantages are derived from several design features of a tachometer generator constructed in accordance with this invention. The core member of the tachometer generator described herein has a pair of extending legs which provides means for accurately positioning the tachometer generator within the associated structure. Additionally, a pair of lead wires have exposed conductor ends which form posts to which the coil wires are attached, thereby eliminating the necessity of providing a separate post component. Further, the lead wires and core member are made integrally with a coil bobbin of the tachometer generator by injection molding the bobbin material around the lead wires and core member which are first inserted into an injection molding die cavity. The lead wires are bent within the bobbin to enhance the mechanical engagement between the lead wires and bobbin.